The present invention relates to audio signal generation apparatus which generate an audio waveform signal by reading out waveform data from a waveform memory having stored therein waveform data representative of waveforms of tones (voices or musical tones). More particularly, the present invention relates to an audio signal generation apparatus which is capable of multifunctionally changing any one of factors, such as a length or duration (reproduction speed (time progression speed)), pitch and formant of a tone, in accordance with a variety of characteristics of any of original tone waveforms (such as a waveform from which a particular tone pitch is extractable, a waveform which is not suitable for extraction therefrom of a particular tone pitch because a plurality of tone pitches are mixed therein, and an indefinite waveform input in realtime) supplied from a variety of tone sources, without influencing the other factors.
There have heretofore been known electronic musical instruments which read out from a waveform memory, waveform data representative of a waveform of a tone and reproduce the tone represented by the read-out waveform data, as disclosed for example in Japanese Patent Application Laid-open Publication No. HEI-9-146555. In the electronic musical instrument disclosed in the No. HEI-9-146555 publication, a plurality of tone generating channels are provided for reproducing tones, and one waveform data (i.e., one set of waveform data) is allocated per predetermined pitch range. Individual sample values constituting the waveform data are compressed and stored in successive addresses of a waveform memory in the order the sample values were sampled. Further, in this electronic musical instrument, there is employed a compression scheme where each sample value is compressed on the basis of a variation from the immediately preceding (i.e., last) sample value, and thus, it is necessary to use the last sample value in order to decode each compressed data. Therefore, in reading out the compressed data, each of the tone generating channels increments a read address one by one.
Each of the tone generating channels calculates a sample value (audio waveform signal) by reading out compressed data from a waveform memory and decoding the read-out compressed data during each sampling time period (i.e., a time period when a DA converter converts a digital value into an analogue value). If a pitch of a tone to be reproduced (hereinafter referred to as a “reproduction tone”) is identical to a pitch of a sampled tone (hereinafter referred to as an “original tone”), the tone generating channel calculates a sample value of a current sampling time period by incrementing the read address by one to read out compressed data and then adding a value of the read-out compressed data to a sample value of the last sampling time period. If the pitch of the reproduction tone is different from the pitch of the original tone, on the other hand, a readout rate of the compressed data is set in accordance with a ratio of the reproduction tone to the pitch of the original tone. Namely, the tone generating channel reads out a plurality of compressed data stored at successive addresses and decodes the plurality of compressed data read out; that is, the tone generating channel sequentially restores a plurality of sample values. Then, a sample value corresponding to the pitch of the reproduction tone is calculated through linear interpolation operations using the restored sample values.
Further, there have heretofore been known a hearing aid equipped with a function for changing (stretching or compressing) a length or duration of a partial portion of voice (a predetermined number of vowels included in a beginning portion of the voice) without changing a pitch of the partial portion of the voice, as disclosed in Japanese Patent Application Laid-open Publication No. HEI-9-312899.
In the aforementioned conventionally-known electronic musical instrument, if the reproduction tone is set at a pitch different from the pitch of the original tone, it would have a different length from the original tone. For example, if the reproduction tone is set at a pitch higher than the pitch of the original tone, the reproduction tone would have a shorter length than the original tone. If the reproduction tone is set at a pitch lower than the pitch of the original tone, on the other hand, the reproduction tone would have a longer length than the original tone. Further, in the case where the reproduction tone is set at a pitch different from the pitch of the original tone in the aforementioned conventionally-known electronic musical instrument, the reproduction tone would have a formant different from that of the original tone. For example, if the reproduction tone is set at a pitch higher than the pitch of the original tone, the reproduction tone would have a higher formant than the original tone. If the reproduction tone is set at a pitch lower than the pitch of the original tone, on the other hand, the reproduction tone would have a lower formant than the original tone. Namely, in the case where the reproduction tone is set at a pitch different from the pitch of the original tone, the reproduction tone would have a difference tone color (timbre) from the original tone.
In the hearing aid disclosed in the No. HEI-9-312899 publication, the aforementioned function is performed using a dedicated circuit (e.g., DSP). In the case where such a dedicated circuit (e.g., DSP) is used, there is a need to store individual sample values constituting waveform data representative of a waveform of a predetermined length until the waveform data is formed. Thus, a memory having a relatively great storage capacity is required. Further, there would occur a time delay from a time when a tone generation start instruction is given (i.e., from a time of voice input) to a time when the waveform data is formed. Further, if a dedicated circuit (e.g., DSP) for performing the aforementioned function is provided, in addition to the aforementioned tone generating channels, in the aforementioned electronic musical instrument, the circuit would increase in size and cost. Particularly, if the number of tones to be generated is increased, the storage capacity of the memory has to be increased, and thus, the inconvenience would become prominent. Besides, because all of the functions are not always used, a considerable waste would result.